This invention relates to developments in a microwave limb sounder comprised of a radiometer operating in the microwave range, typically between 100 and 200 GHz, for the study of the upper atmosphere.
A microwave limb sounder (MLS) has been under development since 1975 for the purpose of providing an instrument to measure trace gases in the upper atmosphere. Basically the instrument switches and compares the spectral noise content between noise received from a horizontal beam by looking out at the limb, i.e., looking out over the horizon, from an aircraft or spacecraft and a reference beam by looking up at cold space at an elevation angle of about 22.degree. above the horizon. The average temperature difference between the two beams over the frequency range of interest is about 30.degree.-40.degree. K, so balancing noise is introduced into the reference beam to reduce this average difference. The spectral features which are superimposed on the average temperature levels are detected by a double conversion synchronous detection receiver. Because the average beam temperatures are balanced, the receiver dynamic range can be kept quite small for maximum sensitivity. The horizon signal beam is formed by an antenna, and the reference beam is formed by a sky plate. The antenna is an offset two-reflector system with a ten-inch television aperture producing an elevation beamwidth of about 0.5.degree. and an azimuth beamwidth of 5.degree.. The antenna subreflector is gimbaled by means of a digital servomotor and encoder, and the antenna beam is roll stabilized by means of a servo-system which receives a roll correction from an internal navigator of a spacecraft or aircraft. The sky plate is a flat reflecting mirror which directs a beam into a radiometer head from cold space at an elevation angle of 22.degree..
The incoming millimeter wave signal from the limb is heterodyned down to a first IF frequency centered at 1.4 GHz in the radiometer head. The 1.4 GHz IF from the radiometer head has a bandwidth of about 100 MHz and this signal is immediately amplified by a 50.degree. parametric amplifier and several stages of low noise transistor amplifiers. The IF amplifiers are housed in a temperature stabilized IF box. The IF signals are then routed to two second heterodyning stages for two second IF conversions. One second conversion takes the IF signal down to a center frequency of about 160 MHz which is then used as an input to a 15 channel filter bank with synchronous detection. The other second conversion is used as an input to a 5 MHz fast Fourier transformer (FFT) digital correlator for fine digital resolution.
The radiometer head is a quasi-optical arrangement of grids and lenses which receives a 11/2 diameter beam and focuses it into a single ended Schottky barrier diode mixer. The local oscillator energy which is provided by a klystron is frequency diplexed into the mixer diode by the radiometer head. Noise generated within the local oscillator in the signal band is filtered and absorbed by the radiometer head diplexer. Finally the radiometer head is provided with power dividers. With these dividers, the local oscillator power is divided between the diode mixer and a phase-lock mixer which is used to provide a stabilization signal to phase lock the klystron. The seventeenth or eighteenth subharmonic of a known frequency in the vicinity of the local oscillator frequency is synthesized to 0.1 MHz accuracy in an X-band frequency synthesizer. This signal is mixed with the local oscillator signal in the harmonic mixer to produce a 2 GHz IF frequency which is then used to phase lock the klystron with a commercial lock box.
A basic requirement for the MLS antenna system was to provide an extremely high efficiency antenna beam of single polarization with the highest possible vertical resolution for low-noise radiometers operating simultaneously at the five millimeter frequencies of 63, 118, 167, 184 and 230 GHz. Besides vertical resolution, the radiometer beams were required to scan slowly in elevation in a programmable manner. Provision also had to be made to provide calibration signals for the radiometers.
Approximately one meter of vertical aperture was available and it was desired that this area be used most efficiently. Ongoing receiver development will probably give rise to higher frequency radiometers in the future so that it was desirable that the MLS antenna system be adaptable to the possible utilization of these new developments as well as possible modification of the present radiometers to other adjacent frequencies.
An object of this invention was therefore to provide an antenna system that meets these basic requirements. Another object was to provide a heterodyne radiometer head to take beam energy from the antenna and energy from a local oscillator and direct them both into a mixer.